Software both commercial and free that allows users to encode audio to ATRAC:
atracdenc Open source ATRAC1 and ATRAC3 Encoder
Sony Simple Burner
Sony .acm codec - integrates with the Windows Audio Compression Manager and encodes to ATRAC3 files wrapped in the WAV container, allowing bitrates of 66, 105 and 132kbps. Keep in mind this is quite old code and probably does not reflect the best quality ATRAC3 can provide.
Sound Forge (starting from what version?)
SONY Cinescore 1.0
Sonic Studio's N-code plugin for professionals
ATRAC Codec Plugin for Sony Media Software (Windows only)
SCEI ATRAC3plus Codec TOOL Russian forum link